1. Field of the Invention
The present invention relates to a high-frequency introducing means for generation of plasma for production of semiconductor devices, electrophotographic photosensitive member devices, image input line sensors, flat panel displays, image pickup devices, photovoltaic devices, and the like. The present invention also relates to a plasma treatment apparatus and a plasma treatment method employing the high frequency introducing means.
2. Related Background Art
Application of plasma for film formation or etching is known in semiconductor device production processes or other processes. For example, plasma CVD apparatuses and plasma CVD methods are used industrially. In particular, the plasma CVD apparatuses employing high frequency of 13.56 MHz or employing microwave of 2.45 GHz are widely used since the apparatuses are capable of treating base members and deposition films whether the material is electroconductive or insulating.
As a conventional example, a parallel plate type high frequency electrode for plasma generation and a plasma CVD apparatus employing the high frequency electrodes are explained schematically referring to FIG. 1. In a reaction chamber 101, a high frequency electrode 103 is held by a high frequency electrode holder 103 made of an insulating material. The high frequency electrode 103 is a flat plate placed parallel to a counter electrode 105. Plasma is generated between the electrodes by an electric field depending on the capacitance therebetween. The generated plasma which is substantially a dielectric forms a sheath between the electrodes the sheath acts chiefly as a condenser equivalent between the plasma and the electrodes or the reaction chamber wall, and in many cases an impedance at this time greatly differs from the impedance before plasma generation. A ground shield 104 is provided around the high frequency electrode 103 to prevent electric discharge between the lateral side of the high frequency electrode 103 and the reaction chamber 101. To the high frequency electrode 103, a high frequency power source 111 is connected through a matching circuit and a high frequency power supplying line 110. On the counter electrode 105 placed parallel to the high frequency electrode, a base member 106 having a flat plate shape is placed. The base member 106 is kept at a desired temperature by a temperature controller (not shown in the drawing).
By using this apparatus, plasma CVD is conducted as described below. The reaction chamber 101 is evacuated to a high vacuum by an evacuation means 107. Then a reaction gas is introduced into the reaction chamber 101 from a gas-supplying means 108, and the gas pressure is kept at a predetermined pressure. High frequency power is applied from the high frequency power source 111 to the high frequency electrode 103 to generate plasma between the high frequency electrode and the counter electrode. Thereby, the reaction gas is decomposed and excited by the plasma to form a deposition film on the base member 106. The frequency of the high frequency power is usually 13.56 MHz. At the frequency of 13.56 MHz, there is a low utilization efficiency of the gas and a relatively low rate of deposited film formation although the discharge conditions can be relatively easily controlled and the formed film has high quality. In view of the above problems, use of higher frequency ranging from about 25 to about 150 MHz has been investigated for the plasma CVD. For example, Plasma Chemistry and Plasma Processing, Vol.7. No.3, (1987) pp.267-273 describes formation of an amorphous silicon (hereinafter referred to as "a-Si") film by decomposing a source gas (silane gas) by high frequency power of 25-150 MHz by means of a parallel plate type glow discharge decomposition apparatus. According to the description, specifically, in formation of an a-Si film by using a frequency selected from the range from 25 to 150 MHz, the film deposition rate by using 70 MHz reaches the maximum of 2.1 nm/sec, which is 5 to 8 times as large as that in the aforementioned plasma CVD at 13.56 MHz. The excitation frequency hardly affects the defect density, the optical bandgap, and the electroconductivity of the obtained a-Si film. The apparatus shown in FIG. 1 is an example of a plasma CVD apparatus suitable for forming a deposition film on a flat plate-shaped base member.
On the other hand, Japanese Patent Application Laid-Open No. 60-186849 discloses an example of a plasma CVD apparatus suitable for film deposition on a plurality of cylindrical base members. It discloses a plasma CVD apparatus employing a microwave energy source of frequency of 2.45 GHz and another plasma CVD system employing a radio frequency energy (high frequency power) source. The RF plasma CVD apparatus employing the radio frequency power source is explained referring to drawings. FIGS. 2A and 2B are schematic sectional views showing the structure based on the RF plasma CVD system described in the above patent application. FIG. 2B is a sectional view taken along the line 2B--2B of FIG. 2A.
In FIG. 2A and FIG. 2B, in the reaction chamber 200, six base member holders 205A are placed concentrically at predetermined space intervals. A cylindrical base member 206 for use in film formation is held by the respective base member holders 205A. In the inside of the respective holders 205A, a heater 240 is provided to heat the cylindrical base member 206 from the inside. The holders 205A are connected respectively to a shaft 231 linked to a motor 232 to be rotated. The cylindrical base members 206 are held also by auxiliary holding members 205B. A high frequency electrode 203 of an antenna type is placed at the center of the plasma generation region, and is connected through a matching circuit 209 to a high frequency power source 211. A high frequency electrode supporting member 230 supports the high frequency electrode. An evacuation pipe 207 having an evacuation valve is communicated to an evacuation mechanism 235 equipped with a vacuum pump. A source gas supplying system 208, which is constituted of gas bombs, mass flow controllers, valves, and so forth, is connected through a gas feeding pipe 217 to gas releasing pipes 216 having a plurality of gas releasing holes. The numeral 233 indicates sealing members.
By using this system, the plasma CVD is conducted, for example, as described below. The reaction chamber 200 is evacuated to a high vacuum by an evacuation mechanism 235. Then a reaction gas is introduced into the reaction chamber 200 from a gas-supplying means 208 through the gas feeding pipe 217 and gas release pipes 16, and the gas pressure is kept at a predetermined pressure level. High frequency power is supplied from a high frequency power source 211 through a matching circuit 209 to the high frequency electrode 203 to generate plasma between the high frequency electrode and the cylindrical base members 206. Thereby the reaction gas is decomposed and excited by the plasma to form a deposition film on the cylindrical base members 206. The plasma CVD apparatus shown in FIGS. 2A and 2B is advantageous in that the gas utilization efficiency is high since the electric discharge space is surrounded by the cylindrical base members.
The aforementioned film formation by using the parallel plate type apparatus at a high frequency power of 25-150 MHz was conducted in a laboratory scale. The applicability in large-area film formation is not mentioned at all. Generally, at a higher excitation frequency, the influence of the standing wave on the high frequency electrode becomes more significant, and in particular when flat plate electrodes are used, complicated two-dimensional standing waves occur, which can make the formation of a uniform large-area film difficult.
When the apparatus shown in FIGS. 2A and 2B is used for formation of deposition film over the entire surface of the cylindrical base members, it is necessary to rotate the cylindrical members. The rotation of the base member causes a drop in the deposition rate to about 1/3 to 1/5 of that of the parallel plate type plasma CVD system. More specifically, in the above apparatus, the deposition film is formed on the cylindrical base members at the position facing the high frequency electrode at the same rate as in the parallel plate type plasma CVD apparatus, whereas at the portion not facing the discharge space, little deposition film formation proceeds. In the above patent application, the frequency of the high frequency power is not mentioned specifically. The inventors of the present application conducted deposition of an amorphous silicon film on the entire surface of a cylindrical base member using the plasma CVD apparatus having a constitution as shown in FIGS. 2A and 2B by rotating the cylindrical base members using SiH.sub.4 as the source gas at a usual high frequency power of 13.56 MHz, and at a pressure of several hundred mTorr where the deposition rate is high but a powdery matter such as polysilane tends to be produced. As a result, the substantial deposition rate was 0.5 nm/s or lower. In production of an electrophotographic photosensitive member having an amorphous silicon layer as a photosensitive layer, the thickness of the photosensitive layer is required to be 30 .mu.m or more. By using the plasma CVD apparatus shown in FIGS. 2A and 2B, at the deposition rate of 0.5 nm/s, the film formation takes as long as 16 hours or more, resulting in poor productivity. In this system also, at a high frequency power of 30 MHz or higher, although the plasma density becomes larger resulting in a larger deposition rate, there is a problem that plasma generation tends to be nonuniform owing to the influence of the standing wave to make extremely difficult the formation of a uniform deposition film on the base member.
In any of the systems using the apparatuses shown in FIG. 1, and FIGS. 2A and 2B, at a frequency power higher than 30 MHz, the plasma density rises to increase the radical generation density and to raise the deposition rate, but the reaction between the radicals is also accelerated to form polysilane which impairs the quality of the deposition film. Although a lower pressure at film formation is effective to retard the formation of polysilane at high radical density, it is difficult to generate and maintain the plasma. In particular, since the impedance after the plasma generation greatly differs from the impedance before the plasma generation, there is a problem that the discharge is interrupted owing to small change of the plasma state and the like by matching to the high frequency.
On the other hand, in the technical field of image formation, the photoconductive material for forming a light-receiving layer of the photosensitive member is required to have a high sensitivity, a high SN ratio (photo current (Ip)/dark current (Id)), absorption spectrum characteristics matching to the spectrum characteristics of the irradiated electromagnetic wave, quick optical response, desirable dark resistance, harmlessness to human bodies in use, and so forth. The harmlessness to human body is particularly important for the electrophotographic photosensitive members built in electrophotographic apparatuses used in offices as a business machine in consideration of direct or indirect contact with human bodies.
An electrophotographic photosensitive member satisfying the above requirements is the one employing amorphous silicon (a-Si). For example, Japanese Patent Application Laid-Open No. 54-86341 discloses a technique regarding an electrophotographic photosensitive member employing a-Si in the photoconductive layer and being excellent in humidity resistance, durability, and electric properties. By using such techniques, the electrophotographic photosensitive member constituted of a-Si has become commercially practical, being improved in electrical, optical, and photoconductive characteristics, use environmental characteristics, and durability, and being capable of improving image quality.
The production of a-Si photosensitive member requires a high level of techniques. In particular, for the electrophotographic photosensitive member, a larger area and a larger thickness of the layer are necessary, so that the uniformity of film formation is an important factor. Therefore, various techniques have been proposed for stable industrial production of high quality a-Si photosensitive member. For example, Japanese Patent Application Laid-Open No. 6-342764 (USP 5540781) discloses an apparatus in which a cathode electrode is divided into several pieces in the direction of the cylindrical base member and high frequency power is applied to the respective cathode pieces to obtain a uniform high-quality deposition film. By using such techniques, the photosensitive member can be obtained stably without variation of the characteristics.
In recent years, however, the electrophotographic apparatuses are required to achieve still higher image quality, still higher speed, and still higher durability. Furthermore, miniaturization of the photosensitive member as well as of the main body of electrophotographic apparatus has become urgently necessary for meeting the needs for space efficiency. Such requirements cannot readily be satisfied by the apparatus constitution disclosed in the above Japanese Patent Application Laid-Open No. 6-342764.
For example, a constitution comprising a cathode of a larger diameter and a plurality of base member placed in the chamber will cause unnecessary increase of the electrode area and may cause difficulty in keeping the uniformity of the electric discharge.
In a constitution in which a plurality of base members are placed around the center cathode electrode, it is often difficult in apparatus constitution to divide the cathode in the axis direction in several pieces and to apply high frequency power to the respective cathode pieces.
Furthermore, the reproducibility of half tone of the image is becoming more important because reproduction of high fine image having high gradation is demanded.
Under such circumstances, high-speed image formation using a smaller-sized photosensitive member makes prominent uneven photosensitivity and uneven image density which do not become problems in the prior art, thereby resulting in density unevenness of half-tone image to impair image quality and resulting in coarseness on a half-tone image appearing such that fine particles are scattered. Thus more uniform photosensitive member is required to solve the above problems.
In such a situation, an apparatus is demanded which is capable of producing a photosensitive member with more excellent uniformity with higher productivity.